Maintenance Troubleshooting
Maintenance interviews check how you think when a machine stops, and also how you stop it from stopping again. Interviewers ask about preventive and predictive maintenance, breakdown handling, root cause analysis, lubrication, spares and the records that keep it all honest. The 70 questions below cover those topics from the shop-floor side. On this page we add the typical round structure, a revision list, and three worked examples with numbers: availability from MTBF and MTTR with the cost of waiting for a spare part, a choice between time-based and condition-based maintenance for a motor, and a 5-why analysis of a bearing that keeps failing. A list of habits that hurt an answer is at the end.
How maintenance troubleshooting interviews usually run
- Maintenance strategies — breakdown, preventive, predictive and total productive maintenance, and when each fits.
- Breakdown scenario — a machine stops and you describe the order of checks and how you restore it fast.
- Root cause analysis — 5-why, fishbone and how you stop a repeat failure.
- Numbers — MTBF, MTTR, availability, spares demand and lubrication quantity.
Concepts and readings to revise first
- Preventive maintenance: time or usage based tasks, and the risk of over-maintaining.
- Predictive maintenance: vibration, thermography, oil analysis and ultrasound readings against trends.
- Failure patterns: early-life failures, random failures and wear-out, and what each means for the plan.
- MTBF and MTTR: what they measure, and how they combine into availability.
- Lubrication: correct grease and oil, quantity, interval and cleanliness.
- Spares: critical spares, lead time, minimum stock and storage.
- Work management: work orders, permits, lockout and tagout, and history records.
- Root cause: 5-why, fishbone and corrective against preventive action.
Showing the first 70 questions free. The complete question bank is available as a PDF below.
Breakdown & Downtime Analysis
Q1A machine shows repeated breakdown despite regular repairs. What is the troubleshooting approach?
AnswerReview the repair history for that machine to check if the same fault keeps recurring or different faults each time. A repeating identical fault points to a root cause never being fixed (only symptoms treated); varying faults point to a systemic issue like poor lubrication or overload.
Q2Unexpected downtime on a line is unusually high. How do you diagnose the cause?
AnswerBreak down downtime by machine, shift, and fault type using maintenance records. This usually reveals one or two dominant contributors (a specific machine or fault category) rather than random failures, letting you focus effort where it matters most.
Q3Breakdown data shows the same fault recurring on a machine. What should be checked?
AnswerConfirm the last repair actually addressed the root cause rather than just resetting/replacing a symptom (e.g., replacing a fuse repeatedly instead of finding why it blows). A proper root-cause analysis (5-Why or fishbone) is needed before the next repair.
Q4A machine fails again soon after a repair or maintenance job. How do you troubleshoot it?
AnswerCheck whether the correct spare part and procedure were used, whether the machine was restarted and monitored properly after the repair, and whether an adjacent, related fault was missed during the original repair.
Q5No clear root cause is found after a breakdown investigation. What is the next step?
AnswerGather more data before the next occurrence — install temporary monitoring (vibration, temperature, current logging) if the fault is intermittent, interview operators for symptoms just before failure, and review any recent process/material changes.
Q6Downtime data is not being tracked properly across the plant. How should this be addressed?
AnswerStandardize a simple downtime logging format (machine, time, fault code, duration) that operators and technicians can fill consistently, and review it in a regular (daily/weekly) meeting so gaps in data collection are caught early.
Q7Multiple machines are failing around the same time period. What should be checked?
AnswerCheck for a common cause affecting all of them — a power quality event, a shared utility (compressed air, cooling water) problem, a raw material/batch issue, or an environmental factor like temperature or humidity spike.
Q8Breakdowns happen mostly during shift change. How do you diagnose this pattern?
AnswerReview handover practices between shifts — an unclear handover of an in-progress issue, inconsistent settings between operators, or machines being restarted without proper checks at shift change are common causes.
Q9A machine breaks down only under a specific load or speed condition. What should be checked?
AnswerReproduce the condition under controlled/monitored conditions if safely possible, and check load-dependent items such as bearing loading, motor current, vibration, and temperature specifically at that operating point rather than at idle.
Q10Emergency (unplanned) maintenance requests are increasing month over month. How do you address this?
AnswerReview whether preventive maintenance compliance has dropped, whether equipment is aging past its reliable service life, or whether operating conditions (speed, load, duty cycle) have increased beyond the original design assumptions.
Preventive Maintenance (PM) Faults
Q11Preventive maintenance is being done but is not effective at reducing breakdowns. What should be checked?
AnswerReview whether the PM tasks actually address the failure modes seen in breakdown data — a mismatch between what is inspected/serviced and what actually fails is the most common reason PM doesn't reduce breakdowns.
Q12The PM schedule is not being followed consistently. How do you troubleshoot this?
AnswerCheck whether the schedule is realistic given available manpower and production windows, and review compliance data to see if specific machines or task types are consistently skipped, which usually points to a resourcing or access issue.
Q13The PM checklist is too generic to catch real faults. What is the fix?
AnswerReview recent failure modes for that equipment and add specific, measurable checks (e.g., actual vibration/temperature readings, not just 'check for noise') so the checklist evolves based on real failure history rather than a generic template.
Q14The PM interval seems too long or too short for a piece of equipment. How do you determine the right interval?
AnswerReview failure/wear trend data for that specific equipment and adjust the interval based on evidence (e.g., a bearing consistently wearing out before the current PM interval means the interval is too long for that component).
Q15The same fault occurs again soon after a PM was completed. What should be checked?
AnswerVerify the PM procedure was followed completely and correctly (not just signed off), and check if the PM task itself is adequate to catch that specific fault mode — the task may need revision, not just repetition.
Q16Spare parts are not available when needed during a scheduled PM. How do you fix this?
AnswerCross-check the PM schedule against the spare parts inventory plan so critical parts are pre-staged before the PM date, and set minimum stock/reorder points for parts used in recurring PM tasks.
Q17PM records are not properly documented, making trend analysis difficult. What is the fix?
AnswerStandardize a PM record format that captures actual readings/conditions found (not just 'OK'), and ensure records are stored centrally and consistently so trends can be reviewed over time.
Q18Different technicians report different findings for the same PM task on the same machine. How do you address this?
AnswerStandardize the PM procedure with clear, specific steps and acceptance criteria, and provide consistent training/calibrated tools so results don't depend on individual technician judgment.
Q19PM tasks are being skipped due to production pressure. How should this be managed?
AnswerEscalate the risk of skipped PM to production and maintenance leadership with data on the consequence of deferred maintenance, and build short, focused PM windows into the production schedule rather than treating PM as optional.
Q20Predictive maintenance data (vibration, oil analysis, thermal) is being collected but not acted on. What is the fix?
AnswerSet up clear alert thresholds and an assigned owner responsible for reviewing predictive data and triggering work orders, since data alone doesn't prevent failures if no action process exists around it.
Lubrication Faults
Q21Lubrication-related failures keep recurring on the same equipment. What should be checked?
AnswerVerify the correct lubricant type, grade, and quantity are being used per the manufacturer's specification, and check that lubrication points haven't changed after any equipment modification or rebuild.
Q22The wrong lubricant grade is suspected to have been used. How do you confirm and correct this?
AnswerCheck the lubricant container/label against the equipment's lubrication chart, and if wrong grade was used, drain/flush and refill with the correct one, then monitor the component closely for any damage already caused.
Q23Over-greasing is suspected of causing bearing or seal damage. What should be checked?
AnswerCheck the actual grease quantity applied against the manufacturer's recommended amount and interval — excess grease can generate heat in bearings and blow out seals, and grease guns/automatic systems should be calibrated to the correct dose.
Q24Lubricant contamination is found recurring in an inspection. How do you diagnose the source?
AnswerSample and analyze the contaminated lubricant to identify the contaminant type (water, dust, metal particles, or mixing with another lubricant), which points to the entry path — a damaged seal, an open fill point, or a wrong top-up product.
Q25Lubrication points are being missed during maintenance rounds. What is the fix?
AnswerCreate a lubrication route map/checklist with all points clearly identified and sequenced, and consider color-coding or tagging points by lubricant type and interval to reduce reliance on memory.
Q26An automatic/centralized lubrication system is not dispensing lubricant. How do you troubleshoot it?
AnswerCheck the reservoir level, pump operation, and for blockages in the distribution lines or injectors — a single blocked line can starve one lubrication point while the system otherwise appears to be running normally.
Q27Grease is leaking visibly from bearing housings. What should be checked?
AnswerCheck for over-greasing first (the most common cause), then inspect seals for wear or damage, and confirm the housing vent (if fitted) is not blocked, which can pressurize the housing and force grease out past the seals.
Q28Oil level in a gearbox or reservoir is dropping without any visible external leak. What is the troubleshooting approach?
AnswerCheck for internal leakage paths (a worn seal allowing oil into an adjacent chamber), consumption through breathers due to overheating/foaming, or a slow leak at a hard-to-see joint such as a drain plug or sight glass.
Q29Lubricant is breaking down (degrading) faster than expected. How do you diagnose the cause?
AnswerSend a sample for oil analysis to check for water contamination, wrong operating temperature, or additive depletion — running consistently above the lubricant's rated temperature range is a very common cause of accelerated breakdown.
Q30A centralized lubrication system shows a blockage. How do you locate and fix it?
AnswerCheck system pressure at progressive stages/injectors from the pump outward to isolate which section is blocked, then clear or replace the blocked line/injector and flush the system before returning it to service.
Bearing, Seal & Mechanical Wear
Q31Bearing failures are recurring on a specific machine. What should be checked?
AnswerCheck lubrication practice first (wrong grease, over/under-greasing), then alignment and balance, and inspect the failed bearing itself for a tell-tale failure pattern (fatigue, contamination, electrical fluting) that points to the specific root cause.
Q32Seal failures are recurring at the same location on a machine. How do you diagnose this?
AnswerCheck shaft surface condition and runout at the seal contact area, correct seal type/material for the operating temperature and media, and installation practice (seals are often damaged during fitting if not handled correctly).
Q33Fasteners (bolts/nuts) are found loose repeatedly during inspection. What is the troubleshooting approach?
AnswerCheck whether the correct torque and tightening sequence was used originally, whether vibration levels are higher than normal at that joint, and whether the correct locking method (thread-locker, lock washer) is being used for the application.
Q34Coupling wear is found frequently during inspections. What should be checked?
AnswerCheck shaft alignment between the coupled machines, since misalignment is the most common cause of accelerated coupling wear, along with confirming the coupling type and rating are correct for the torque and speed being transmitted.
Q35Belts or chains are wearing out faster than the expected service life. How do you diagnose this?
AnswerCheck tension (both over- and under-tensioning accelerate wear), alignment between pulleys/sprockets, and for contamination (oil, dust) on the belt/chain surface that accelerates degradation.
Q36Shaft wear is found at the bearing seating area. What is the likely cause and fix?
AnswerThis is usually caused by bearing inner-ring creep from a loose fit, often due to repeated bearing changes without restoring the original fit tolerance. The shaft may need sleeving/repair, and the fit should be re-verified on reassembly.
Q37Oil analysis shows gearbox wear debris. How do you interpret and act on this?
AnswerCheck the debris type and size against wear-limit guidelines (fine wear versus larger fatigue particles indicate different severity), and use trending across multiple samples to decide whether continued monitoring or a teardown inspection is warranted.
Q38Housing or bore wear is causing bearing looseness in its seat. What should be checked?
AnswerMeasure the actual bore diameter against specification to confirm wear, and consider a sleeve/bushing repair or housing replacement rather than repeatedly fitting new bearings into an oversized bore, which will fail again quickly.
Q39A newly replaced bearing fails prematurely. What should be checked?
AnswerCheck installation practice (correct tools, heating method, and fit), lubrication applied at installation, alignment after reassembly, and whether the bearing was actually stored and handled correctly before fitting (contamination or shock damage before installation is common).
Q40Wear debris is found in an oil filter during a routine check. How do you respond?
AnswerIdentify the debris material (using a magnet, visual inspection, or lab analysis) to trace it to a specific component type (bearing, gear, or seal), and increase monitoring frequency until the source is confirmed and addressed.
Alignment & Vibration
Q41Abnormal vibration is recurring even after maintenance work. What is the troubleshooting approach?
AnswerTake a vibration reading and compare its frequency spectrum against baseline/history — different fault types (imbalance, misalignment, bearing defect, looseness) show distinct frequency signatures that point directly to the cause.
Q42Machine alignment is repeatedly lost after being corrected. How do you diagnose this?
AnswerCheck for a soft foot condition, thermal growth between cold and running conditions, or a foundation/baseplate issue causing the alignment to shift once the machine reaches operating temperature or load.
Q43Vibration levels are increasing gradually over several weeks. What should be checked?
AnswerTrend the vibration data over time and correlate with maintenance/operational events — a gradual increase typically indicates progressive wear (bearing, coupling) rather than a sudden fault, and trending helps plan intervention before failure.
Q44Imbalance is suspected after a rotor or fan was cleaned. How do you confirm and correct it?
AnswerTake a vibration reading focused on the running-speed frequency, which is the signature of imbalance, and perform a field balancing correction if confirmed — uneven cleaning or debris buildup on one side of a rotor/fan is a common cause after cleaning.
Q45Resonance is suspected at a certain operating speed. How do you diagnose this?
AnswerPerform a run-up/coast-down vibration test to identify if vibration peaks sharply at a specific speed (resonance) rather than scaling steadily with speed (imbalance), and check if that speed coincides with the equipment's natural frequency.
Q46Coupling misalignment is found during a vibration check. What is the correction procedure?
AnswerPerform a proper alignment procedure (dial indicator or laser alignment) covering both parallel and angular offset, and re-check under normal operating temperature since cold alignment can differ from running alignment due to thermal growth.
Q47A soft foot condition is detected on a motor or pump base. How do you correct it?
AnswerMeasure the gap under each foot with a feeler gauge or dial indicator while loosening bolts one at a time, and correct with proper shimming rather than forcing the machine down with excessive bolt torque, which can distort the frame.
Q48Vibration only appears under load, not at no-load/idle. What should be checked?
AnswerCheck for load-dependent issues such as coupling misalignment (which often shows up more under torque), a bent shaft, or a structural/foundation weakness that only becomes apparent once the machine is loaded.
Q49A new or rebuilt piece of equipment vibrates more than the old one it replaced. What is the troubleshooting approach?
AnswerCheck installation quality first (alignment, foundation, balance) rather than assuming a manufacturing defect, and compare vibration readings against the equipment's acceptance criteria/ISO vibration severity standard for its class.
Q50A vibration analysis report identifies a fault, but no action is being taken. How should this be addressed?
AnswerEstablish a clear process linking vibration reports to work order generation with assigned priority and owner, since analysis without a follow-up action process provides no reliability benefit.
Spares & Reliability Metrics (MTBF / MTTR)
Q51A spare part fails early (infant mortality) shortly after installation. What should be checked?
AnswerCheck installation practice and handling (many early failures are installation-induced, not part defects), confirm the part's storage history and shelf life, and verify it is the correct genuine part for the application.
Q52A machine's MTBF (Mean Time Between Failures) is decreasing over time. How do you diagnose this trend?
AnswerBreak down MTBF by failure mode to see if one specific issue is driving the decline (e.g., aging electrical components) versus a general equipment condition decline, and check if operating conditions have changed.
Q53MTTR (Mean Time To Repair) is increasing for the same type of fault. What should be checked?
AnswerCheck for spare parts availability delays, technician skill/training gaps, unclear repair procedures, or access/safety issues that are extending repair time beyond what the fault itself would normally require.
Q54Critical spare parts are not available when a breakdown occurs. How should this be addressed?
AnswerPerform a criticality analysis to identify which spares justify stocking based on failure frequency and downtime impact, and set appropriate stock levels and reorder points for those critical items specifically.
Q55Non-genuine or duplicate/local spare parts are suspected of causing failures. What should be checked?
AnswerCompare the failed part against OEM specification (material, dimensions, rating), and review the procurement process to confirm genuine or approved-equivalent parts are being sourced for critical applications.
Q56Spare parts inventory count does not match actual usage records. How do you fix this discrepancy?
AnswerConduct a physical stock audit against the inventory system, and review the parts issue/transaction process to identify where records are not being updated correctly (unrecorded withdrawals are a common cause).
Q57A refurbished or rebuilt spare part fails faster than a new one would. What should be checked?
AnswerReview the refurbishment process and quality checks used, and confirm critical wear components were actually replaced (not just cleaned/reused) during the rebuild, since incomplete rebuilds are a common cause of early failure.
Q58Long lead-time spares are causing extended downtime during failures. How should this be managed?
AnswerIdentify long-lead items during criticality analysis and either stock them proactively, arrange a local repair/machining option, or establish a shared/pooled spare arrangement with sister plants or suppliers.
Q59Reliability metrics (MTBF, MTTR, availability) are tracked but not linked to maintenance planning. What is the fix?
AnswerBuild a regular review process where these metrics directly drive maintenance strategy decisions (e.g., moving a component from reactive to preventive/predictive strategy based on its failure trend).
Q60Repeated failures occur on newly installed or commissioned equipment. What should be checked?
AnswerReview the commissioning checklist and initial alignment/settings, since early-life failures on new equipment often trace back to installation errors rather than equipment defects — this is a normal part of the reliability 'bathtub curve' if not addressed.
Maintenance Process & Root Cause
Q61A maintenance job is completed but does not solve the underlying root cause. How do you fix the process?
AnswerIntroduce a formal root-cause analysis step (5-Why, fishbone) for repeat and significant failures before closing the work order, rather than accepting 'fault cleared' as sufficient for recurring issues.
Q62A safety device trips frequently during normal operation. What should be checked?
AnswerConfirm whether the trip is a genuine, correct response to an actual condition (in which case the underlying condition needs fixing) versus a nuisance trip from a miscalibrated or degraded safety device — never simply bypass a safety trip to stop the nuisance.
Q63A maintenance inspection finding from a previous round was missed or not actioned. How do you prevent recurrence?
AnswerReview the finding-to-work-order process to ensure every inspection finding is logged, prioritized, and tracked to closure, rather than relying on inspectors to remember or informally report follow-ups.
Q64A work order is closed, but the reported fault reappears shortly after. What should be checked?
AnswerVerify the technician's completion notes against what was actually done, and check whether the original symptom was fully understood before the repair — closing a work order without confirming the fault is truly resolved under real operating conditions is a common gap.
Q65An operator-reported fault does not match what maintenance finds during investigation. How do you reconcile this?
AnswerInterview the operator for more specific detail on when/how the fault appears (it may be intermittent or load/condition-dependent), and consider observing the equipment during the reported condition rather than only inspecting it at rest.
Q66Maintenance history for a piece of equipment is not readily available. How should this be fixed?
AnswerConsolidate equipment history into a single accessible system (CMMS) covering all repairs, PM records, and parts used, since fragmented or paper-based records make trend analysis and root-cause work very difficult.
Q67Repairs done by a particular technician seem to fail more often than others. How should this be addressed?
AnswerReview training and competency for that technician against the specific task type, and check whether standard procedures/job aids are available and being followed, rather than assuming individual fault without investigation.
Q68Maintenance backlog (pending work orders) is increasing continuously. What should be checked?
AnswerReview whether backlog growth is due to insufficient manpower/planning capacity, work being generated faster than it can be closed, or an approval/parts bottleneck slowing down work order completion.
Q69Root cause analysis (RCA) is not being performed consistently for significant failures. How is this fixed?
AnswerDefine clear criteria for which failures require formal RCA (based on cost, safety, or repeat-failure criteria), assign ownership, and track RCA completion and resulting corrective actions as part of the maintenance KPI review.
Q70Maintenance KPIs are not improving despite ongoing effort. What should be reviewed?
AnswerCheck whether the KPIs being tracked actually reflect the improvement actions being taken (a mismatch between effort and metric is common), and ensure data quality feeding the KPIs is accurate before concluding the effort itself is ineffective.
Three worked fault examples
Example 1: “A press has an MTBF of 450 hours and an MTTR of 6 hours. What is its availability, and what happens if each repair has to wait 24 hours for a spare?”
- Availability = MTBF ÷ (MTBF + MTTR) = 450 ÷ (450 + 6) = 450 ÷ 456 = 98.7%.
- If a spare is not in stock and the repair waits 24 hours, MTTR becomes 6 + 24 = 30 hours.
- Availability = 450 ÷ (450 + 30) = 450 ÷ 480 = 93.75%.
- The loss is about 5 percentage points, and nothing in the machine changed. The wait for the part did it.
- For a spare used about twice a year with an 8-week lead time, expected demand in the lead time is 2 × 8 ÷ 52 = 0.31. On a simple Poisson estimate, holding one spare covers about 96% of cases.
Improving MTTR with a stocked critical spare, a ready kit and a clear procedure is often cheaper than trying to raise MTBF. Availability is a ratio, so both sides count.
Example 2: “A 22 kW fan motor is changed every 6 months by the preventive plan. A new vibration route shows its velocity rising from 2.0 to 3.1 to 4.4 mm/s over three months, and an infrared check shows the drive-end bearing 18 °C hotter than the twin motor. What do you do?”
- Treat the trend as real. Both measurements agree, so the cause is probably a bearing, and the rise from 2.0 to 4.4 mm/s in three months is fast.
- For this size of machine, many users read about 2.8 mm/s as the start of the alert range and about 7.1 mm/s as the danger range, but use the limits in your site standard.
- Plan the bearing change at the next stop instead of running to the danger level, and take a vibration spectrum so the fault type is confirmed, for example a bearing defect frequency or looseness.
- Check the greasing first. Over-greasing and misalignment can raise both vibration and temperature.
- Compare cost as an illustration: changing a ₹8,000 bearing set every 6 months costs ₹16,000 a year. If condition monitoring costs ₹6,000 a year and the bearing lasts about 2 years, the yearly cost is about ₹4,000 + ₹6,000 = ₹10,000.
Predictive methods pay on critical, costly machines with a measurable warning period. On cheap, non-critical items running to failure, or simple time-based replacement, may be the better choice.
Example 3: “The drive-end bearing of a pump motor has failed three times in 14 months. How do you find the root cause?”
- Why did the bearing fail? Overheated, the grease was burnt and the track was discoloured.
- Why overheated? Too much grease, from a technician who gave it about 40 g at each visit.
- Why so much? The task card says “grease until it comes out” with no quantity, and the grease relief was blocked.
- Why no quantity? Nobody had worked out the amount, and the card was copied from another motor.
- Why was that not caught? Failures were recorded as “bearing failed”, with no cause, so the pattern was never reviewed.
Work out the right amount. A common rule of thumb for re-greasing is G = 0.005 × D × B in grams, with D the outside diameter and B the width in mm. For a 6310 bearing (110 mm and 27 mm) that is 0.005 × 110 × 27 = about 15 g, so 40 g was well over double. Put the quantity and interval on the card, clear the grease relief, retrain, and record failure causes. Always confirm the figure against the bearing or motor maker’s advice.
Common mistakes that cost candidates the job
- Replacing a failed part and closing the job without asking why it failed.
- Greasing by feel, with no quantity or interval written down.
- Applying the same preventive interval to every machine regardless of its criticality.
- Quoting MTBF without MTTR, or vice versa, when asked about availability.
- Skipping lockout and tagout to save time on a quick fix.
Frequently asked questions
What is the difference between MTBF and MTTR?
MTBF is the average running time between failures, and MTTR is the average time taken to repair and return the machine to service.
When is predictive maintenance worth it?
When the failure is costly, the machine is critical, and the fault gives a measurable warning such as rising vibration or temperature.
How many whys should I ask?
As many as it takes to reach a cause you can fix, often about five. Stop at a system cause, not at a person.
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